Best to change on the correct intervals.
Checking the specific gravity tells you if the antifreeze properties are okay, but not the corrosion inhibitors, which are used up in time. Antifreeze should be completely renewed generally every 2, 5 or 10 years, depending upon the type used.
That said, if the history was unknown but the antifreeze looked clean, and the sg was good, I wouldn't worry too much for now. If it looked opaque and smelled of old antifreeze, or the sg indicated low antifreeze content, I would probably change it.